In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex® cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the membrane skin thickness, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than are less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability. One of the immediate challenges that need to be addressed in CA polymer membrane is achieving higher selectivity with equal or greater permeability. Another issue is the plasticization of CA polymer that leads to swelling and to an increase in the permeability and a decrease in the selectivity of CA membrane.
High-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole were developed to combine high selectivity and high permeability together with high thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on the use of glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer as represented by the membrane structure swelling and significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Most recently, McKeown et al. reported the synthesis of a new type of polymer, termed polymers of intrinsic microporosity (PIMs), with a randomly contorted molecular structure, bridging the void between microporous and polymeric materials. The rotational freedom of these PIM materials has been removed from the polymer backbone. These polymers exhibit properties analogous to those of conventional microporous materials including large and accessible surface areas, interconnected micropores of less than 2 nm in size, as well as high chemical and thermal stability, but, in addition, possess some favorable properties of conventional polymers including good solubility and easy processability for the preparation of polymeric membranes. Polymeric membranes have been prepared directly from some of these PIMs and both the liquid and gas separation performances have been evaluated. Membranes from PIMs have shown exceptional properties (e.g. extremely high gas permeability) for separation of commercially important gas pairs, including O2/N2 and CO2/CH4. The exceptionally high permeability of gases arises from the rigid but contorted molecular structures of PIMs, frustrating packing and creating free volume, coupled with chemical functionality giving strong intermolecular interactions. Two published PCT patent applications provide further detail: WO 2005/012397 A2 and WO 2005/113121 A1, both applications incorporated by reference in their entireties. Membranes from PIMs, however, have much lower selectivities for commercially important gas pairs, such as O2/N2 and CO2/CH4, although their gas permeabilities are significantly higher than those of commercial polymeric membranes from glassy polymers such as CA, polyimides, and polyetherimides.
The present invention involves a UV-cross-linking method to further improve the separation performance of membranes containing PIMs, particularly the selectivities for separation of gas pairs such as O2/N2 and CO2/CH4 through the formation of interpolymer-chain-connected cross-linked networks.
Environmental concerns have led to the decreases in the permissible levels of sulfur in hydrocarbon fuels. Sulfur in refinery streams, e.g., feedstocks, is present in a number of different forms, including aliphatic and aromatic compounds. Sulfur, however, tends to be concentrated in the higher boiling fractions, mainly in the form of aromatic heterocycle compounds such as benzothiophenes and dibenzothiophenes. Sulfur compounds in the gasoline and diesel fuels are converted to SOx during combustion, which not only results in acid rain, but also poisons catalysts in catalytic converters for reducing CO and NOx.
Currently, deep desulfurization of gasoline and diesel fuels is attracting wide interest because of the increasingly stringent environmental regulations on sulfur concentration in gasoline and diesel fuels. The sulfur level in diesel fuels must be reduced to 15 wppm (0.0015% by weight) by 2006 in the United States, and to 10 wppm by 2010 in the European Union. The sulfur level in gasoline must be reduced to 30 wppm by 2006. Further, lower sulfur limits are expected for highway diesel fuels and also for non-road diesel fuels in the near future. Diesel fuel is considered to be one of the promising liquid hydrocarbon fuels for producing H2 for use in automotive and portable fuel cells due to its high energy density. However, the sulfur compounds in the fuel and H2S produced from them by the hydrocarbon reforming process are poisons to reforming and shift catalysts as well as the electrode catalysts. Thus, sulfur concentrations in the fuel needs to be reduced to less than 1 wppm for proton exchange membrane fuel cell and less than 10 wppm for solid oxide fuel cell.
Refiners have employed catalytic hydrodesulphurization processes to reduce sulfur in gasoline and diesel fuels. Substantial advances have been achieved in new catalyst developments and new reactor technologies along with improved processes for producing low-sulfur gasoline and diesel fuels. However, severe operating conditions (e.g. high temperature, high pressure and high hydrogen consumption) are required for hydrodesulphurization to achieve ultra-deep desulphurization of gasoline and diesel fuels. The production of ultra low sulfur gasoline and diesel at a sustainable cost, and available in quantities that will meet the demands of U.S. drivers, is tentatively the single most significant and costly challenge faced by the refining industry. Therefore, new, more cost efficient technologies such as membrane separation (see U.S. Pat. No. 6,896,796 B2 and US 2005/0067323 A1), adsorption, extraction, oxidation, and bioprocesses need to be developed rapidly to provide alternatives to refiners in the 2006-2010 timeframe.
Membrane separation technology provides a new viable efficient approach for the separation of organic mixtures in the petrochemical field. Desulphurization of gasoline and diesel fuels by pervaporation membrane process is a newly emerged technology. Pervaporation is a process that separates mixtures of liquids by partial vaporization through a non-porous membrane. The membrane acts as a sulfur selective barrier between two phases, the liquid phase feed and the vapor phase permeate. A pervaporation process involves contacting a gasoline or diesel fuel feed with a membrane having sufficient flux and selectivity to separate a sulfur deficient retentate fraction from a sulfur enriched permeate fraction. Sulfur deficient retentate fractions are useful and can be added directly into the gasoline or diesel pool, as appropriate. Sulfur enriched permeate fractions need further treatment by conventional processes. Compared to traditional sulfur removal processes such as a hydrodesulfurization process, a pervaporation membrane process offers a number of potential advantages, including high selectivity (separation efficiency), lower energy consumption, lower operating costs, and simple operation.
However, there have been only a few reports of the selective permeation of sulfur-containing compounds using a membrane separation process most recently. See US 2005/0067323 A1; U.S. Pat. No. 6,896,796 B2 and U.S. Pat. No. 6,649,061 B2.
In 2004, W. R. Grace & Co.'s Davison Membranes initiated a joint marketing effort with CB&I for Davison Membranes' S-Brane™ membrane-based gasoline desulfurization process. S-Brane™ is a pervaporation process using polyimide polymer membrane that selectively removes sulfur-containing molecules from fluid catalytic cracking (FCC) and other naphtha hydrocarbon streams. This S-Brane™ membrane separation process, however, has not been commercialized up to now mainly due to the level of sulfur selectivity that would require the sulfur to be removed with a high stage cut. Therefore, the naphtha recovery in the retentate is low, leading to high octane loss.
Thus, the development of advanced materials for the preparation of new generations of pervaporation membranes with improved sulfur selectivity and high permeability for deep desulfurization of gasoline and diesel fuels is still critically required.
One objective of the present invention is to design and develop a new generation of low cost and high performance UV-cross-linked polymer membranes that are capable of reducing the sulfur content of gasoline to <30 wppm and diesel fuels to <10 wppm with less octane loss than the current state-of-the-art desulfurization polymer membranes. The resulting technology also provides platforms for other pervaporation membrane processes such as for bioethanol recovery from the fermentation products in the biomass conversion to bioethanol process.